1. Field of the Invention
The present invention relates to multi-directional stereo multiple recording and reproducing apparatus and a reproducing record disc, and particularly to a recording apparatus adapted to record a modulated subchannel signal on a record disc at a constant acceleration in order to decrease a noise produced by a drop-out which is caused by a harmonic distortion of a main-channel signal during the reproduction of a record disc.
2. Description of the Prior Art
There have hitherto been proposed various types of multi-directional stereo multiple recording apparatus and record discs. A so-called universal matrix system (hereinafter referred to as a UM system) is known as one system of the recording apparatus. The UM system has advantages in that the compatibility between monoral and 2-channel stereo systems is superior, the fidelity of original sound field reproduction and its sound image positioning are good and the freedom of loud-speaker disposition is high.
The basis of the prior art UM system will be described with reference to FIG. 1. In FIG. 1, reference numeral 1 designates a magnetic recording and reproducing apparatus in which a magnetic tape is recorded with a left-front signal S.sub.LF, a right-front signal S.sub.RF, a left-back signal S.sub.LB and a right-back signal S.sub.RB which represent a sound field. These four signals are derived from microphones disposed at positions corresponding to a left-front loud-speaker L.sub.F, a right-front loud-speaker R.sub.F, a left-back loud-speaker L.sub.B and a right-back loud-speaker R.sub.B which are disposed perpendicularly intersecting each other in a listening space surrounding a listener 20 at, for example, a record disc reproducing apparatus shown in FIG. 2.
These four signals S.sub.LF, S.sub.RF, S.sub.LB and S.sub.RB derived from the magnetic recording and reproducing apparatus 1 are applied to an encoder 2 to derive therefrom main-channel signals T.sub.L and T.sub.R and sub-channel signals T.sub.T and T.sub.Q which are expressed by the following equation (1).
______________________________________ T.sub.L = 0.924 S.sub.LF .angle. 22.5.degree. + 0.383 S.sub.RF .angle. 67.5.degree. + 0.383 S.sub.RB .angle. - 67.5.degree. + 0.924 S.sub.LB .angle. - 22.5.degree. T.sub.R = 0.383 S.sub.LF .angle. - 67.5.degree. + 0.924 S.sub.RF .angle. - 22.5.degree. + 0.924 S.sub.RB .angle. 22.5.degree. + 0.383 S.sub.LB .angle. + 67.5.degr ee. (1) T.sub.T = 1.414 S.sub.LF .angle. 135.degree. + 1.414 S.sub.RF .angle. 45.degree. + 1.414 S.sub.RB .angle. - 45.degree. + 1.414 S.sub.LB .angle. - 135.degre e. T.sub.Q = 1.414 S.sub.LF .angle. + 90.degree. + 1.414 S.sub.RF .angle. - 90.degree. + 1.414 S.sub.RB .angle. + 90.degree. + 1.414 S.sub.LB .angle. - 90.degree ______________________________________
The main-channel signals T.sub.L and T.sub.R from the encoder 2 are phase-compensated by a filter and phase-compensating circuit 3 and the sub-channel signals T.sub.T and T.sub.Q are filtered thereby. The phase-compensated main-channel signals T.sub.L and T.sub.R are applied to a recording equalizer 4 having RIAA characteristics, while the sub-channel signals T.sub.T and T.sub.Q are fed through pre-emphasis circuits 10 and 11 to angle modulators 6 and 7 to angle-modulate a carrier signal of a frequency f.sub.c.
Thus angular-modulated signals are expressed as modulated sub-channel signals T.sub.T ' and T.sub.Q '. These signals T.sub.T ' and T.sub.Q ' are applied to a mixer 5 for being added with the main-channel signals T.sub.L and T.sub.R to form two multiplex signals T.sub.L + T.sub.T ' and T.sub.R + T.sub.Q '. These multiplex signals T.sub.L + T.sub.T ' and T.sub.R + T.sub.Q ' are supplied through a recording amplifier 8 to a 45.degree. -45.degree. record disc cutter 9, which is used to cut a prior art stereo record, thereby to engrave a recording lacquer disc 12 with the signal T.sub.L + T.sub.T ' on one wall of its sound groove and with the signal T.sub.R + T.sub.Q ' on the other wall thereof.
Thus, the sound groove of the lacquer disc 12 is recorded on its one groove wall 13L with the main-channel signal T.sub.L and the modulated sub-channel signal T.sub.T ' and on its other groove wall 13R with the main-channel signal T.sub.R and the modulated sub-channel signal T.sub.Q ' as shown in FIG. 1.
Further, in another prior art example, the sub-channel signals T.sub.T and T.sub.Q from the pre-emphasis circuits 10 and 11 are applied to a sum and difference circuit 29 shown by dotted lines in FIG. 1 to produce signals T.sub.T + T.sub.Q and T.sub.T -- T.sub.Q which are applied to the angular modulators 6 and 7 to angular-modulate the carrier signal. Thus, the angular modulators 6 and 7 produce signals (T.sub.T + T.sub.Q)' and (T.sub.T -- T.sub.Q)' which are added with the main-channel signals T.sub.L and T.sub.R at the mixer 5 to produce two multiplex signals T.sub.L + (T.sub.T + T.sub.Q)' and T.sub.R + (T.sub.T -- T.sub.Q)'. These signals are supplied to the cutter 9 to engrave the sound groove of the recording lacquer disc 12 at its one wall with the signal T.sub.L + (T.sub.T + T.sub.Q)' and at its other wall with the signal T.sub.R + (T.sub.T - T.sub.Q)'.
An apparatus for reproducing the sound of the thus recorded disc for the multidirectional stereo reproduction will next be described with reference to a systematic diagram shown in FIG. 2. A record disc 15 produced by the lacquer disc 12 is placed on a turntable 14. A signal picked up from the record disc 15 by a pickup device 16 is applied to a preamplifier 17 to derive therefrom the main-channel signals T.sub.L and T.sub.R which are fed to a decoder 22 through a reproducing equalizer 18 having characteristics reverse to those of the recording equalizer 4. The modulated sub-channel signals T.sub.T ' and T.sub.Q ' derived from the preamplifier 17 are applied through a band pass filter 19 for separating the main-channel signal components therefrom to an angular demodulator 21 to reproduce the sub-channel signals T.sub.T and T.sub.Q. These signals T.sub.T and T.sub.Q are supplied to the decoder 22 together with the main-channel signals T.sub.L and T.sub.R. As a result, the decoder 22 produces signals similar to audio signals S.sub.LF, S.sub.RF, S.sub.LB and S.sub.RB which exhibit a sound field recorded on the 4-channel tape. These signals S.sub.LF, S.sub.RF, S.sub.LB and S.sub.RB are supplied through an amplifier 23 to the left- and right-front loud-speakers L.sub.F and R.sub.F and left- and right-back loud-speakers L.sub.B and R.sub.B which are arranged in 2-2 disposition.
Further, in that case that the sum and difference circuit 29 is inserted in the sub-channel as another embodiment shown in FIG. 1, a sum and difference circuit 30 having a characteristic reverse to that of the circuit 29 of FIG. 1 is connected between the decoder 22 and the angular demodulator 21. The signals (T.sub.T + T.sub.Q)' and (T.sub.T - T.sub.Q)' from the band pass filter 19 are applied to the angular demodulator 21 to derive therefrom demodulated signals T.sub.T + T.sub.Q and T.sub.T - T.sub.Q which are applied to the sum and difference circuit 30 to produce the original signals T.sub.T and T.sub.Q. These signals T.sub.T and T.sub.Q are applied to the decoder 22.
When considering a noise, caused by the record disc itself during the reproduction of a multi-directional record disc, the signal-to-noise ratio of the main-channel signals T.sub.L and T.sub.R is good, while that of the sub-channel signals T.sub.T and T.sub.Q is not so good. It is considered that the inferior signal-to-noise ratio of the latter is caused by imperfect engraving of a carrier level signal on the lacquer disc 12, or a noise dependent upon the fineness of the particles of the record material or the granularity of a record disc, and the like. Accordingly, when a 4-channel reproduction is carried out by way of example, signal-to-noise ratio during the reproduction is greatly affected by that of the sub-channel signals T.sub.T and T.sub.Q. Particularly, a phenomena similar to a drop out is liable to occur at the inner groove of a record disc. This is because, at the inner groove of the record disc, a harmonic distortion (particularly a secondary harmonic distortion component) caused by reproducing a higher frequency signal component of the main-channel signal becomes great. Meanwhile, when the carrier level is decreased on account of reproducing loss or the like and the harmonic distortion of the main-channel signal is relatively increased in modulated band, a phenomena similar to the drop out occurs. For this reason, in the cutting system, when the level of the main-channel signal is increased in higher frequency range, its level is made low or the carrier level is made high to avoid a noise similar to the noise caused by the drop out. However, in order to reduce the level of the high channel signal if the excessive application of a limiter is used, no high channel is produced and hence no signal is reproduced with high fidelity. Further, as set forth above, even if at the inner groove of the record disc, the carrier level is made high, the curvature radius of crest and trough of record becomes small at the inner periphery of record upon reproduction. Thus, the reproducing stylus can not achieve reproduction with high fidelity. Thus, a curvature overload occurs. The high frequency components of modulated signals are lowered, so that the effect of a carrier level control (hereinafter referred to as a CLC) is low.
When reproducing the record disc as described above, the carrier level is limited on account of the curvature overload of a reproducing pickup stylus or the like. In FIG. 3, if the abscissa represents frequency in KHz and the ordinate represents the velocity level of the cutting stylus in mm/.sub.sec, a modulating level is not increased upon reproduction, for example, under the condition that the carrier level upon cutting is taken above a straight line 24 as described later. Then, it results in an abnormal reproducing condition or the curvature overload is apt to occur. This curvature overload is apt to occur at the inner groove rather than at the outer groove. The quantitative expression thereof will be given in the following manner.
In the case where the carrier frequency is 30 KHz, the radius of a record disc is 60 mm and the revolution is 33 r.p.m., the waveform of the carrier is determined by the following factors, that is, the wave length .lambda. which is expressed as follows: ##EQU1## where f is the frequency, R is the radius of a record disc, and N is the number of rotation of the record disc; the displacement amplitude a which is given as follows: ##EQU2## where V is the velocity amplitude of the signal; and r is the curvature radius of modulated signal expressed as follows: ##EQU3## Thus, the following values are obtained. EQU .lambda. = 7.0 .mu., a = 0.185 .mu., r = 6.8 .mu.
When the stylus is considered, the curvature radius of the stylus chip is generally at most 7 .mu. due to the limitation of its manufacture and reducing the abrasion of record, and becomes substantially coincident with the curvature radius r of modulated signal of the expression (4). Thus, the range of velocity amplitude which avoids the curvature overload is about 35.4 mm/sec.
Meanwhile, when the velocity limiting line 24 at which the curvature overload appears is obtained, if a groove speed is taken as S, the following relation is obtained. EQU f.lambda.=S (5)
by substituting the equation (5) for the equation (4), the following equation isobtained. ##EQU4## This is rewritten as follows: ##EQU5## If both members of the above equation (7) are multiplied by .omega., the following equation is obtained. ##EQU6## The above is rewritten as follows: ##EQU7## In this case, the groove speed S is given as follows: ##EQU8## where D is the diameter of the record disc and T is the period. If the equation (10) is substituted for the equation (9), the following equation is given. ##EQU9## The above is rewritten as follows: ##EQU10## Thus, the straight line 24 shown in FIG. 3 is obtained.
The cutting velocity of a main-channel signal is expressed normally with RIAA characteristics as in FIG. 4. The cutting velocity v.sub.M of a signal level of 1 KHz is 22.3 mm/sec. In this case, second and third harmonic distortions D.sub.2 and D.sub.3 are calculated as follows: ##EQU11## It will be apparent from the above that as the groove speed S becomes small, the tracing distortion increases abruptly. This results from the fact that if a velocity for cutting the groove is decreased, a recording wavelength becomes short to make the waveform of the sound groove sharp and hence a contact point of the sound groove wall with the reproducing stylus is greatly shifted from the bottom end of the reproducing stylus. If this harmonic distortion appears in the modulated band to disturb a modulated signal, the drop out phenomenon occurs. In this case, however, if the amount of a harmonic component to cause the disturbance is small as compared with the level of the modulated signal, the drop out may not occur.
In the above described example, when the cutting velocity v.sub.M of the signal level of 1 KHz is 22.3 mm/sec, the second and third harmonic distortions are shown by straight lines 25 and 26 in FIG. 3. Of course, in this case, the harmonic components of the main-channel signal are contained in the modulated band to produce the distortion components which are shown by the straight lines 25 and 26 in frequency and velocity level. The straight line 25 shows a characteristic of substantially 3rd power of frequency and the straight line 26 substantially 5th power of frequency. A dotted straight line 27 shows the velocity level of a prior art constant speed recording.
In FIG. 3, if a main-channel signal of, for example, 10 KHz is taken into consideration, the frequency of the second harmonic distortion becomes 20 KHz. Therefore, the second harmonic distortion caused by the component of the main-channel signal having a frequency more than 10 KHz is contained in the modulated band and similarly the third harmonic distortion caused by the component of the main-channel signal having a frequency more than 6.7 KHz is contained in the modulated band. As apparent from the figure, the second harmonic distortion is greater than the third harmonic distortion so that the third harmonic distortion may be almost neglected. When a number of music signals are recorded on the record, there is nearly no component of the main-channel signal more than 15 KHz, so that it can be considered that the harmonic distortion is distributed mostly in a range less than 30 KHz.
It will be noticed from the foregoing that the modulated signal of 30 KHz or less is easily disturbed by the harmonic distortion of the main-channel signal. It may be considered that when the high frequency component of the main channel signal is mixed into the carrier, by raising the carrier level the S/N ration of the mixed signals is improved. However, such a method is apt to be affected by the curvature overload.